The present invention relates to an optical write-in head and an image forming apparatus using the same and, more particularly to, an optical write-in head which writes information sent from a row of light sources selectively emitting light corresponding to image information to a photosensitive substance using a gradient index rod lens array, an image forming apparatus using the same, and a method for inspecting the image forming apparatus.
Conventionally, as an optical write-in head for writing optical information sent from a row of light sources, such as an LED array or a liquid crystal display (LCD) shutter array, emitting light corresponding to image information to a photosensitive substance on a photosensitive drum or a photographic printing paper sheet, there are mainly available the following two types of gradient index rod lens arrays.
(I) Such a rod lens array that each rod lens (hereinafter called a lens element) thereof has angular aperture θ in a range of 20 to 23°. The rod lens array of type (I) has the following merits. (a) It has a high coupling efficiency and light load on a light-source row's emitted light amount and a photosensitive-substance sensitivity. If the light source row is an LCD shutter array, the light-source row's emitted light amount is equal to a light amount of back light which is emitted from the LCD shutter array and has passed therethrough. (b) It has a relatively short conjugate length TC, so that a relevant apparatus can be downsized. (c) It has the relatively short conjugate length TC and so is not easily influenced by an inclination of an array of the lens elements.
(II) Such a rod lens array that each lens element thereof has angular aperture θ within a range of 10.5 to 12°. The rod lens array of type (II) has the following merits. (d) It has a high basic resolution, so that a clear photographic pint-out can be obtained. (e) It has a relatively large focus depth and so can easily accommodate an attachment error, eccentricity of the photosensitive drum, etc.
The rod lens array of type (I), however, has a problem that it is difficult to obtain a clear image free of stripes in printing at a high recording density of approximately 1200 dpi (dot/inch). The reasons are as follows. (i) Generally, the resolution becomes higher as the angular aperture becomes smaller. In the case of type (I), since the angular aperture θ is a large value of 20 to 23°, a not so high resolution level can be attained, so that it is impossible to obtain a resolution of 1200 dpi. (ii) Although the lens element has a larger curvature of field as the angular aperture is larger, the angular aperture θ is the large value of 20 to 23°, thus giving a large resolution fluctuation in a period of a lens diameter/radius.
(iii) The angular aperture θ has a large value of 20 to 23°, so that the focus depth is small. Accordingly, the resolution is significantly deteriorated by a defocus of approximately ±20 μm, which is inevitable due to an attachment error, eccentricity of the photosensitive drum, etc. The term “defocus” here refers to that of the lens element with respect to the centerline interconnecting the light source and the photosensitive drum.
The rod lens array of type (II) has the following problems. (i) Load is heavy on a light-source row's emitted light amount and a photosensitive substance's sensitivity, so that a full-scale cooling mechanism is necessary depending on a printing speed. This is because the angular aperture θ has a large value of 10.5 to 12.0° and, therefore, the coupling efficiency of this type of rod lens array is only ¼ of that of the type (I) of rod lens array. (ii) Since the conjugate length TC of this type of rod lens array is nearly double that of the type (I) of rod lens array, the apparatus becomes large. (iii) Since the conjugate length TC is large, an inclination of the array of the lens elements has a large influence, so that stripe-like irregularities are liable to appear locally.
Thus, in printing at a high recording density of approximately 1200 dpi, a requirement for obtaining a clear image free of stripes cannot be satisfied by type (I) or (II) of rod lens array, both of which have advantages as well as disadvantages.
In an electro-photographic-type image forming apparatus using the conventional lens array, stripe-like irregularities may appear in an output image owing to fluctuations in optical characteristics of each of the lenses of the lens array. The stripe-like irregularities are caused not by a low absolute value of the resolution but by a large difference from the surroundings in resolution. These irregularities are distinct particularly when such stripes appear in a period of 1 to 5 mm because the width/period of the stripes that can be easily recognized by the naked eye is about 0.5 to 10 mm.
In evaluation of a picture quality of an image formed by the image forming apparatus, conventionally, a light source array, a lens array, and a photosensitive substance are assembled into an apparatus, which is then operated actually to carry out photo-reaction and development processes, so that a resultantly formed image is checked for irregularities. In the present situation, it is possible to know whether there are stripe-like irregularities caused by the lens array only after the apparatus is assembled.
Therefore, if, as result of evaluation, some stripe-like irregularities are found in the image and its picture quality obtained is decided to be poor, the apparatus needs to be disassembled to identify a cause. That is, it is necessary to analyze the factor of the appearance of the stripe-like irregularities on whether it is ascribed to the lens array or the light source array. Furthermore, if the lens array is decided to be defective, a non-defective lens array must be used to assemble the apparatus again. This makes the inspection process more time consuming and labor intensive, thus resulting in poor productivity of the apparatus.